This invention relates generally to voltage control circuitry suitable for charging an output capacitor used to periodically supply an output current pulse. The invention is particularly suited for use in a battery powered device intended to be implanted in a patient""s body for supplying a current pulse to stimulate body tissue.
Implantable devices for stimulating body tissue are known in the prior art. For example, see U.S. Pat. No. 5,193,539 by Schulman, et al.; assigned to the same assignee as the present application. Such devices typically store energy in an output (or xe2x80x9cstimulationxe2x80x9d) capacitor which is periodically discharged to supply an output current pulse to stimulate targeted tissue. The energy source primarily discussed in U.S. Pat. No. 5,193,539 for charging the capacitor is comprised of an external source for generating an alternating magnetic field. The alternating field energy is inductively coupled to an internal power supply circuit for producing a voltage for charging the stimulation capacitor. Unfortunately, such prior art devices require that a patient remain in very close proximity to the external source to enable the devices to continue to operate. For example, such devices are typically limited to operating for only several seconds to a minute or so without requiring additional energy from the external source.
More recent implantable devices have incorporated rechargeable batteries capable of operating for prolonged periods, in excess of one hour and up to many days, without requiring additional energy from an external power source. This difference in independent operating duration leads to a qualitative difference in the utility of the device and how effectively the devices can serve a patient.
In such battery operated implantable devices, it is very desirable to control the energy transfer from the battery to the output capacitor in a manner to minimize energy inefficiencies, i.e., unproductive energy losses, while also retaining the ability to control the amplitude, duration, and frequency of output current pulses supplied by the output capacitor to an impedance load, e.g., body tissue.
The present invention is directed to voltage control circuitry driven by a battery for producing a voltage for charging an output capacitor. The voltage control circuitry converts the battery voltage VBAT to a charging voltage VUPC based upon programmed parameters and the value of the output voltage VCOMPL measured at one terminal of the capacitor.
In many tissue stimulation applications, it may be medically efficacious to produce a high amplitude current across a high magnitude tissue impedance between output electrodes. This situation necessitates the application of a high amplitude charging voltage VUPC to the output capacitor. In accordance with the preferred embodiment, the voltage control circuitry includes an up/down voltage converter for deriving the charging voltage VUPC based on the battery voltage VBAT. The conversion can be implemented by known techniques including, for example, placing a plurality of converter capacitors in parallel across a voltage source for charging, and then switching the capacitors into a series configuration to produce a voltage equal to some multiple of the source voltage. Alternatively, the converter capacitors can first be placed in series across the voltage source and then switched to a parallel configuration to produce a voltage equal to some fractional value of the source voltage.
In accordance with the preferred embodiment, the output capacitor voltage is sampled at a specific point in time relative to each discharged current output pulse to generate the signal VCOMPL.
In accordance with a significant feature of the preferred embodiment, the charging voltage VUPC is compared with the output capacitor voltage VCOMPL to determine a clock rate used to convert VBAT to VUPC. For example, the clock rate can be off, slow, or fast depending upon the charge condition of the output capacitor. This feature is useful to conserve energy and avoid premature depletion of battery energy.
More particularly, in accordance with the preferred embodiment, the output voltage, sometimes referred to as the xe2x80x9ccompliancexe2x80x9d voltage VCOMPL, is sampled to determine its final xe2x80x9cdroopxe2x80x9d at the end of an output current pulse. If the final droop value is lower than a certain threshold (xcex94VLOWER), then the voltage converter switches to increase the converter charging voltage VUPC. On the other hand, if the final droop value is above a certain threshold (xcex94VUPPER), then the voltage converter switches to reduce the value of the voltage VUPC. This feedback action maintains the output capacitor voltage within an acceptable operating range to provide sufficient energy to produce an efficacious output current pulse for stimulation without causing unproductive energy loss, e.g., heat.
Exemplary embodiments of the invention intended for tissue stimulation are configured to be accommodated in a small implantable housing preferably having an axial dimension of less than 60 mm and a lateral dimension of less than 6 mm so as to be readily injectable. Devices in accordance with the invention preferably operate from a 2.4 to 4.5 V battery to provide stimulation output current pulses having a controllable amplitude of between 5 microamps and 20 milliamps, a controllable pulse width of between 10 microseconds and 2 milliseconds, a controllable repetition rate of between 1 pulse per second and 1000 pulses per second (pps).